1. Field of the Invention
This invention relates to a heat-sealing apparatus associated with a continuous packaging machine, adapted to use shrink film, The apparatus heat seals a continuous tube of film surrounding filled trays spaced within it, in the transverse direction, between each tray. More particularly, the apparatus provides a predictable and consistent transverse bead-seal on the shrink film, which allows the resulting sealed film bags to be shrunk in a consistent and uniform manner around the trays, providing virtually leak-free packages.
2. Description of Prior Art
Plastic film sealed around objects provides an inexpensive packaging alternative to individually made and specifically designed packaging such as paper cartons. The film, in effect, becomes an individual plastic packaging container in the form of a sealed `bag`.
Various types of film are used in this way, and various methods used to form the individual plastic bag. The film may be `stretch` film, `shrink` film, or `hard` film (which neither stretches readily, nor is adapted to heat shrinking significantly). Each type of film requires somewhat different procedures to form it into a bag, and the bags formed may be quite different. Hard film may be made into relatively `loose` bags surrounding the packaged object. An example is the type of bag that is used for crackers in food dispensing machines. Shrink and stretch film, by contrast, usually are formed into bags contoured tightly around the packaged object.
Many different items are packaged in these plastic bags. The item or items may be already individually packaged itself, such as in cans or boxes, and a bag formed to enclose several of the cans or boxes. The object to be packaged thus consists of several prepackaged items. Often, items may be placed in a tray and then surrounded by a bag. Food items such as chicken parts for instance are commonly `trayed` and then bagged in plastic film. Here the object to be packaged is an open tray containing food items.
As noted, the methods of making the bag is quite varied. It may be made from film previously extruded into a robe. The tube may then be made into individual bags into which the item to be packaged is placed, then sealed at one or both ends, either thermally or with some sort of closure device. Shrink film, for instance, is used for packaging food in this way. However, a major thrust in bag type packaging is the formation of bags from a continuous roll of film sheet rather than from pre-extruded tube, where the items to be packaged may often be disposed in a tray. Here, bags are formed continuously around the item or trayed items. This is achieved by first forming the sheet into a continuous tube, around continuously supplied items or trays by continuously sealing the film edges together, longitudinally, below the item or tray, and then sealing (or cutting then sealing) the tube in the direction transverse to the direction of the moving tube and trays, into individual bags. The tube is generally formed horizontally, around the continuously fed items or trays. The items or trays in the tube are moved along, with the tube, by a horizontal conveyor. The direction in which the film, the tube formed from the film, and the item or trays travel is commonly referred to as the `machine` direction or `longitudinal` direction. The direction of the sealing and cutting of the formed tube tube between each tray is commonly referred to as the `transverse` or `cross` direction.
The general mode of continuous operation, film sheet-to-tube-to-bag, is common to many continuous packaging machines, using different type of film. However, not only are there differences in how the tube is formed, but there are major differences in the type of seal, and in how the longitudinal and transverse seals are made. There is then the question of if and how the formed bag is then operated on to finish the packaging operation. Since the method of sealing and the type of seal varies, the nature of the resulting bag will vary. Typically it will depend on the type of film--hard, stretch or shrink. Three common forms of seal are the `fin`-seal where the film sheet is pressed together to form a fin, the `bead`-seal, where film is pressed together, melts and forms a bead or beads (sometimes referred to as welding), and an `overwrap`-seal, where cut ends are pressed around the item or trayed items and sealed. Fin-seals may also be subsequently overwrapped.
A continuous packaging machine of the above described general mode of operation, i.e. sheet-to-tube-to-bag, is described in U.S. Pat. No. 5,125,216 (Redaelli). In this particular case, the disclosure appears to describe formation of a loose bag from hard film, and the item is not necessarily in a tray. It uses fin type seals both in the machine and transverse direction. Fin-seals are generally not particularly tight seals, and are generally not suitable for packaging moisture containing items.
Stretch film may also be fin sealed. Commonly, overwrap type of closure rather than true sealing is also used with stretch film. As noted, fins themselves may also be overwrapped and further loosely sealed. Items such as chicken parts disposed in foamed plastic trays are extensively packaged using the general mode of `sheet-to-tube-to-bag` packaging, using stretch film. A longitudinal fin-seal in the machine direction is formed continuously, underneath the trays, from stretched film stretched around the trays. After fin-sealing the fin is overwrapped underneath the tray, heating to affix the fin underneath. In the transverse direction, the film may not be sealed immediately, but merely cut between individual packages, and overwrapped by folding the film under the tray, and then heat sealing the overwrap. The `bags` (using the term bag rather loosely) formed in this way from stretch film however, are highly subject to leakage of any liquid from items within the package. Furthermore, while stretching reduces the amount of film required, overwrapping excess film increases the amount of film used.
Shrink film is used extensively using continuous sheet-to-robe-to-bag type machines to package non-food objects alone, or where the object is a food or other item or items previously enclosed in, for instance, cans, boxes or bottles). Shrink film has not been utilized to any great extent in continuous packaging of food items where the film comes into direct contact with the food, particularly food containing moisture such as chicken parts disposed in open trays. Because of the general chemical and physical nature of shrink film, in contrast to stretch film, shrink film is more adaptable to sealing the film by welding into bead-seals rather than mere fin-seals. Bead-seals can only readily be formed from films when not under tension, as stretch film often is in sealing operations. Typically, heated sealing members such as bars or rolls press two sheets of film together, melting them and forming a bead on one or both sides of the sealing-bars or sealing-rolls. In the longitudinal direction, rolls are commonly used, and in the transverse directions bars are commonly used. In the longitudinal direction a bead may be formed, continuously, above sealing-rolls located underneath the trays, the bead forming between the sealing-rolls and the trays. The bead is also severed from excess film under it by the heat of the sealing-rolls. In the transverse direction, it is necessary to make two bead-seals, to seal the tube ends around both the tray ahead of, and the tray before, the sealing-bars. Here bead-seals form either side of the sealing-bars, one or both of which are heated, and the bars also heat-sever between the beads. In the longitudinal direction, the film heat-severed below the single bead is waste. In the transverse direction, where the heat-severing of film is between two beads, there is no excess film to be cut off.
With shrink film, the whole operation of forming bead-seals and then shrinking the loose formed bag to form a tight bag around trays requires a degree of precision that overwrap sealing of stretch film around trays does not require to quite the same extent or at least in the same way. The loose bag must be precisely located around the tray to be able to be shrunk around the tray in a consistent way. It is partly the type of precision required which has precluded any real penetration of shrink film into the field of continuous packaging of moisture-containing food-in-trays.
Various attempts have been made to increase the precision of the sealing operations, required for satisfactory operation of continuous packaging machines of the sheet-to-tube-to-bag type, even for stretch and hard film, as well as for shrink film. The ideal, for any type of film, is to achieve consistent and uniform seals. By consistent is meant similarity from seal to seal, and by uniform is meant uniformity along the seal itself. Consistency becomes more difficult with line speed variations which may be necessitated by unavoidable variation in the size of the items to be packaged, such as size variation of chicken parts in trays. Of course with quite different objects, the size of the final package in both longitudinal and transverse direction may obviously be quite different. An ideal packaging machine should instantly adapt to and compensate for both inline size variations and to major changes in the size of objects to be packaged.
The nature and quality of a seal of any type will depend primarily on three factors: the temperature at which, pressure at which, and dwell-time during which, the seal is made. For trayed items, even if these factors are controlled adequately, the position of the seal with respect to the trays requires control. Attempts to improve the consistency of the seal itself have been made. Redaelli, previously referred to, for instance, attempts to make both longitudinal and transverse seal more consistent and uniform by means which increase the pressure of the sealing in proportion to the film speed. In this case the items were not disposed in trays and hard film appears to be used, but the same principles apply. He notes, in passing, that temperature is too difficult to control sufficiently rapidly because of thermal inertia. He makes no attempt to control the dwell-time during which the sealing occurs.
U.S. Pat. No. 4,063,400 (Millervoi) describes a packaging machine of the sheet-to-tube-to-bag type, specifically for shrink film, which attempts to control the uniformity of the transverse seal. In this case, it is the dwell-time which is the subject of control, rather than the pressure, as in Redaelli. The range of possible transverse seal dwell-times is increased by allowing vertically moving sealing elements also to move horizontally in the machine direction, travelling with the speed of the packaging objects, by having the sealing elements be housed in a horizontally movable framework (referred to here as a `seal-head`). The length of horizontal travel can be adjusted to alter the dwell-time while the machine is running. While the dwell-time can be manually adjusted, there is no attempt to provide automatic accommodation to continuous variation is line speed, and package size. Nor is there any attempt to automatically position the seal with respect to the height and length of the package.
There remains a need in continuous shrink packaging machines of the film sheet-to tube-to-bag type, where items are disposed in trays, for high levels of consistency and uniformity of the transverse seals, together with controlled positioning of those transverse seals with respect to the trays. In addition, there is a need to accommodate linespeed fluctuations, as well as a need to be able to readily adapt to different size trays and different size items within those trays. Fulfilling this need would allow the loose formed bags to be shrunk to produce more uniform and consistent packages, without constant manual adjustments.